The definition of the situation is a fundamental concept in symbolic interactionism advanced by the American sociologist W. I. Thomas. It involves a proposal upon the characteristics of a social situation (e.g. norms, values, authority, participants' roles), and seeks agreement from others in a way that can facilitate social cohesion and social action. Conflicts often involve disagreements over definitions of the situation in question. This definition may thus become an area contested between different stakeholders (or by an ego's sense of self-identity).

A definition of the situation is related to the idea of "framing" a situation. The construction, presentation, and maintenance of frames of interaction (i.e. social context and expectations), and identities (self-identities or group identities), are fundamental aspects of micro-level social interaction.